A conventional AC motor driving apparatus includes a DC power supply which supplies DC power, an inverter which converts the DC power from the DC power supply to AC power and supplies the AC power to an AC motor, and a control device for them. As the DC power supply used in this case, there are various types depending on application of the AC motor. For example, when the AC motor is a motor for driving an electric rolling stock, a DC wire is the DC power supply. In addition, when the AC motor is an industrial motor such as a servomotor, AC power from a power supply system is rectified by a converter to supply DC power.
Meanwhile, AC motors having various characteristics have been put into production. Among them, there is an AC motor having two types of rated outputs, short-time rated output and continuous rated output. In such a case, the short-time rated output of the AC motor is set so as to have a very high value as compared to the continuous rated output. The reason is that, for example, in the case where the AC motor is operated to accelerate or decelerate, when the AC motor operates at the short-time rated output only for a relatively short time such as during acceleration or deceleration, it is possible to reduce the time required for speed change. In this case, it is necessary to select the DC power supply and the inverter according to the short-time rated output, and, accordingly, a power supply facility also needs to have a capacity which can tolerate the short-time rated output.
However, when an actual operation is performed, peak power is equivalent to the short-time rated output, but the average power may be decreased on a time average basis. Thus, a problem arises that the prepared power supply facility is not effectively used. And, due to the preparation of the power supply facility according to the short-time rated output, it may be difficult to introduce the apparatus.
For solving such a problem, various techniques have conventionally been developed. For example, in the conventional technique in Patent Document 1 described below, a power compensator including a capacitor which stores power and a step-up/down circuit which converts a voltage level is provided, and when power or a current used by an inverter or a converter exceeds a predetermined value, energy is discharged from the power compensator to suppress a current peak of a DC power supply. In addition, in the conventional technique in Patent Document 2 described below, when the voltage or current of a DC bus bar connected to an inverter exceeds a predetermined value, power of a power compensator is discharged or absorbed.
When such a conventional technique disclosed in Patent Document 1 or 2 is applied, power stored in the power compensator is discharged while demand for peak power occurs, thereby enabling AC motor driving to be realized at the short-time rated output over the limitation on the capacity of the power supply facility.    Patent Document 1: Japanese Patent No. 4339916    Patent Document 2: Japanese Laid-Open Patent Publication No. 2005-328618